The present invention relates generally to intracavity doubled single mode lasers and more particularly to an intracavity doubled single mode laser implementation which exhibits high efficiency in converting light from a fundamental frequency to a doubled, visible frequency. The invention is particularly well suited for efficiently producing visible light in the blue frequency range.
In the prior art, a variety of intracavity doubled lasers have been developed. However, many of these frequency doubled lasers suffer a so called "green noise" problem which limits their usefulness in a number of applications. More specifically, the green noise problem introduces amplitude noise (i.e. variation in the intensity of the output beam at the doubled frequency) which is believed to be due to gain competition between multiple modes present in the laser's resonant cavity in combination with a longitudinal mode coupling phenomenon which is a consequence of a nonlinear doubling process.
One popular approach in solving the green noise problem is to eliminate the multiple modes in the laser light by using a single longitudinal mode (hereinafter SLM) to excite a non-linear material and thereby eliminate longitudinal mode coupling so as to obtain a single, doubled output frequency. However, as will be seen hereinafter, certain problems have been encountered with regard to use of an SLM laser in efficiently producing light at particular frequencies such as, for example, that of blue.
Referring to FIG. 1, as mentioned, one approach for producing a stable output beam at a doubled frequency in accordance with the prior art is to excite a nonlinear material with a single longitudinal mode. In a specific implementation (the physical elements of which are not shown), a type II birefringent nonlinear material cooperates with a polarizing element so as to reject all but a desired fundamental wavelength. FIG. 1 illustrates, in this implementation, the well known orientation of the polarizing element's polarization axis 10 with respect to the ordinary axis 12 and extraordinary axis 14 of the birefringent type II nonlinear material. Specifically, ordinary axis 12 and extraordinary axis 14 are each oriented at an angle of 45.degree. with respect to polarization axis 10. In this way, a Lyot filter is formed in the laser's cavity which serves to discriminate against all but one desired fundamental frequency while providing low insertion loss for the desired mode. It should be appreciated that, in order to achieve useful output power, this implementation is most appropriate in applications where a reasonably efficient nonlinear type II doubling material is available for use at the wavelengths of concern. For example, green output light at 532 nm may be produced with relative efficiency from a 1064 nm fundamental wavelength using type II KTP. Unfortunately, however, the efficient production of blue output light using type II configured materials is complicated by the fact that infrared transitions which, when doubled, yield blue light tend to have a net optical gain around one-tenth of those transitions which yield green light when doubled.
In order to compensate for this lower optical gain, it is desirable to use much more efficient doubling materials. At first appearance, it would seem that certain doubling materials such as, for example, potassium niobate (KNbO.sub.3) would serve well in a high efficiency conversion role since this material possesses an effective doubling efficiency for blue interactions which is greater than thirty times that of KTP in green interactions. However, KNbO.sub.3 will only phase match blue interactions in a type I configuration such that it is not useful in the orientation of FIG. 1. Therefore, type I doubling materials must be used in other geometries, as will be described immediately hereinafter. These other geometries are typically inefficient as a result of additional intracavity components which constrain the laser to SLM operation. As an example, intracavity etalons are currently the component of choice for defining SLM operation. In most instances, etalons introduce excessive and undesirable insertion losses which significantly reduce the laser's visible light output power. Moreover, intracavity etalons do not constrain the polarization state of the laser in a precise way, thereby reducing the etalon's effectiveness in cases where the gain material being used does not exhibit sufficient gain anisotropy.
Other implementations may also employ type I doubling materials. Examples include "twisted mode" cavity designs, in which a combination of intracavity waveplates is used, and ring lasers. These implementations normally embody complex design considerations and geometries. Ring lasers, in particular, are susceptible to mode-hopping behavior (i.e., noisy).
As will be seen hereinafter, the present invention provides a heretofore unseen and highly advantageous intracavity geometry and associated method which utilizes a type I doubling material in a way that provides stable SLM operation and high efficiency.